I am aware that people with blood type A has A antigens on the surface of their red blood cells and B antibodies in their plasma whereas people with blood type O do not have A or B antigens on the surface of their red blood cells but they have both the A and B antibodies in their plasma.

I learnt that transfusion will only work if a person who receives blood has a blood group that doesn't have any antibodies against the donor blood's antigens, otherwise agglutination will occur.

People with blood type O is able to donate blood to people with blood type A. Why is it possible? Wouldn't the A antibodies from blood type O react with the A antigens in A blood person when O blood is transfused into a person with blood type A?

No each blood type should have the same number of antibodies, but transfusions contain a limited number of them. The danger comes from 'rejection' of the blood whereby antibodies in the blood of the recipient attack the new blood cells and kill them before they become useful. Hence O-type (with no receptors) are the universal donors because no antibodies can bind to them.However they can only receive other O-type because their plasma contains IgM antigens for both A and B receptors and will quickly kill them off.